The present invention concerns a method and device for the sputtering onto substrates of a layer of particles sputtered from a cathode surface using gas glow discharge and which provides for the formation of a dense homogenous plasma over a large volume, making possible the sputtering of layers onto substrates when the substrates are placed at distances of 100 to 500 mm from the cathode.
Coating thin layers by cathode sputtering is a well-known process. Usually, cathode sputtering is preferred over other methods of coating layers, such as vacuum evaporation, in that cathode sputtering offers high reproducibility, layer coating in arbitrary directions, such as from top to bottom, and allows mixture and alloy compositions from a sputtered cathode to be transformed into layer form, etc. However, classic diode sputtering is inefficient and time consuming, due to the high gas pressures necessary for maintaining a glow discharge. Consequently, methods and means utilizing a magnetic field for reduction of working pressure during sputtering have been proposed. These systems are based on U.S. Pat. No. 2,146,025 issued to Penning (1939). Another solution was proposed in U.S. Pat. No. 3,616,450 issued to P. Clarke (1971). In accordance with that patent, the path of electrons in a sputtering device is elongated by a cylindrical hollow anode placed in an axial magnetic field and by a sputtering cathode formed in the shape of a hollow cylinder, accommodated coaxially with the anode, outside the magnetic field. However, a more successful solution was magnetron discharge in accordance with U.S. Pat. No. 3,878,085 issued to J. Corbani (1975) and U.S. Pat. No. 4,166,018 issued to J. S. Chapin (1979). In accordance with those patents a closed tunnel of magnetic field lines of force is formed, the path of electrons in this tunnel is elongated, ionization is increased, and sputtering is accelerated. See also, J. L. Vossen and W. Kern, Thin Film Processes, pp. 76-140 (Academic Press, New York, 1978).
In practice, it is sometimes necessary during layer sputtering that when material is condensing on the substrate, at the same time the substrate must also be bombarded with charged particles of suitable energy, such as positive ions. This method of coating is called ion plating. Before being used with sputtering, ion plating was used with vacuum evaporation. An example is evaporating with an electronic beam in accordance with U.S. Pat. No. 4,197,175 issued to Moll et al. (1980). Ion plating during magnetron sputtering is well-known from U.S. Pat. No. 4,116,791 issued to Bizega (1978). A substrate is placed on an electrode which is supplied with a negative voltage relative to the vacuum chamber, while a magnetron cathode is placed opposite the substrate and is also supplied with a negative voltage relative to the vacuum chamber. The biased electrode with substrates thereon attracts ions from the magnetron discharge, and thus ion plating occurs. In accordance with U.S. Pat. No. 4,426,267 issued to W. Munz et al. (1984), a method and device for coating three-dimensional bodies are provided. In accordance with that method, bodies intended for coating move between two magnetron cathodes having a common glow discharge, in the space between them. It is possible to supply the substrates with negative bias for ion plating.
One drawback of the above-mentioned methods of ion plating during magnetron sputtering is that the ionization current extracted by the biased substrates quickly drops as the distance between the substrate and the magnetron cathode increases. Typically, when the distance between the substrate and the cathode is 20 to 50 mm, the ionization current is too low for successful ion plating. Furthermore, the plasma between the pair of cathodes cannot be sustained if the cathodes are separated by large distances, thus making it impossible to use the above-mentioned methods for ion plating remote or large objects. It is possible to increase the plasma density at greater distances from a magnetron cathode however. One way to do this is by means of arc discharge in a hollow cathode, from which electrons are extracted for plasma ionization. This system is disclosed in U.S. Pat. No. 4,588,490 to J. J. Cuomo (1986). However, such a solution is complicated and expensive.
A certain increase of the charged particles' current on substrates is observed with a planar magnetron of the "unbalanced" type; see B. Window and N. Savvides, J. Vac. Sci. Technol., A4:196-202 (1986). In this type of magnetron, some magnetic field force lines which radiate from the periphery of the sputtered cathode approach each other and at greater distances recede from each other. Substrates placed in a magnetic field above the cathode are subjected to a greater bombardment by charged particles than with the classic "balanced" magnetron.
It is possible to attain higher ionization currents on substrates than is possible with unbalanced magnetrons by application of a double-sided discharge in accordance with Czechoslovakian author's certificate No. PV 8659-88 of S. Kadlec, J. Musil and W. D. Munz. In this device, there is formed an intense magnetic field between the cathode and the substrates, and the discharge between the cathode, substrate and anode is maintained by processes on the cathode and on the substrates. High induction of the magnetic field concentrated in the space between the cathode and substrates guarantees maintenance of a dense plasma and guarantees that the density of ionization current flowing on the substrates does not drop with increasing cathode distances up to about 200 mm.
A drawback of unbalanced magnetron and double-sided discharge is that the plasma and density of ionization current on the substrates are not sufficiently homogenous across the magnetic field's lines of force. In addition, substrates are inevitably placed directly in the magnetic field and this field is therefore affected by the magnetic properties of substrates. Consequently, it is practically impossible to use the same device for both weak magnetic and ferromagnetic substrates.
It is well known from plasma physics that a relatively dense and homogenous plasma can be maintained using a multipolar magnetic field. See, for example, R. Limpaecher and K. R. Mac Kennzie, Rev. Sci. Instrum. 44:726 (1973). Plasma is generated in such a system by emission of electrons from glowing cathodes and at the same time the plasma is maintained by a multipolar magnetic field formed by permanent magnets placed around the whole chamber oriented with alternating polarity. The purpose is to produce a steady plasma with high spatial homogeneity in the central part thereof where the magnetic field is very low.
Besides plasma generation by emission of electrons there is a well-known method of plasma generation by absorption of microwaves to decompose gases such as SF.sub.6, and use of the decomposition products to etch substrates. French patent No. 25-83-250 (1986) issued to Y. Arnal, J. Pelletier, and M. Pichot discloses methods and devices which teach how to combine such a microwave-generated discharge and multipolar containment, or "holding" to provide a more homogenous and denser plasma, so as to increase the homogeneity of the plasma-produced reactive gas, and provide a more homogenous generation of radicals and therefore an increase of etching homogeneity and anisotropy, as stated in the work of Y. Arnal, et al., Appl. Phys. Lett.. 45:132 (1984). However the purpose of multipolar holding as used in the above-mentioned cases is different than for maintaining plasma for ion plating during sputter deposition, where a direct-current glow discharge occurs between anode and sputtered cathode.